Intergalactic Stars

Did you know that a few percent of all stars are traveling alone through intergalactic space, no longer a part of any galaxy? Gravitational interactions between stars or between stars and black holes can occasionally accelerate a star to galactic escape velocity so that it is thrown (eventually) into intergalactic space. When the star first enters intergalactic space, the view of your home galaxy would be pretty remarkable, but eventually (eons later, of course) there would be very few naked eye objects in your night sky. Just moons and planets, meteors, aurora, comets, the zodiacal light, and maybe a galaxy or two. Anything else would require a telescope. And an observer, of course.

The first evidence for intergalactic stars came from the detection of diffuse light between galaxies (Zwicky 1952). Much later, intergalactic planetary nebulae were detected in the Fornax galaxy cluster (Theuns & Warren 1997). More recently, intergalactic red giant stars have been detected in the Virgo galaxy cluster using the Hubble Space Telescope (Ferguson et al. 1998).

The Fornax cluster lies about 62 million light years distant, and the Virgo cluster 54 million light years distant. Have any intergalactic stars been detected near our Milky Way galaxy? Brown et al. (2005) discovered the first hypervelocity star, SDSS J090745.0+024507, a 20th-magnitude star in the constellation Hydra. Though it is just 160,000 light years from the center of our galaxy, it is moving away from the Galactic center at an astonishing radial velocity of 709 km/s. Even though this one-dimensional radial velocity1 is only a lower limit to the star’s true 3D space motion, it is far and away fast enough to escape our Milky Way galaxy altogether. Gaia will probably be able to measure this runaway star’s proper motion in right ascension and declination, thus allowing a determination of the true space velocity of SDSS J090745.0+024507 relative to the Galactic center.

Several more hypervelocity stars have been discovered since 2005. One of them, US 708, a 19th-magnitude white dwarf in Ursa Major, is exiting our galaxy at a velocity of at least 1200 km/s! This makes it the fastest on record (Geier et al. 2015).

1The observed one-dimensional radial velocity as seen from Earth is corrected for the Earth’s rotation and motion around the Sun, and the Sun’s motion around the center of the Milky Way galaxy to determine the galactocentric radial velocity.